A conventional boiling water reactor (BWR) includes a pressure vessel containing a nuclear fuel core immersed in circulating coolant, i.e., water, which removes heat from the nuclear fuel. The water is boiled to generate steam for driving a steam turbine-generator for generating electric power. The steam is then condensed and the water is returned to the pressure vessel in a closed loop system. Piping circuits carry the heated water or steam to the steam generators and turbines and carry recirculated water or feedwater back to the pressure vessel that contains the nuclear fuel.
The BWR includes several conventional closed-loop control systems that control various individual operations of the BWR in response to demands. For example a control rod drive control system (CRDCS) controls the position of the control rods within the reactor core and thereby controls the rod density within the core which determines the reactivity therein, and which in turn determines the output power of the reactor core. A conventional recirculation flow control system (RFCS) is used to control core flow rate, which changes the steam/water relationship in the core and can be used to change the output power of the reactor core. These two control systems work in conjunction with each other to control, at any given point in time, the output power of the reactor core and thereby establish the electrical power output of the electric generating plant. A turbine control system (TCS) controls steam flow from the BWR to the turbine based on pressure regulation or load demand.
The operation of these systems, as well as other conventional systems, is controlled utilizing various monitoring parameters of the BWR. Some monitoring parameters include core flow and flow rate effected by the RFCS, reactor system pressure, which is the pressure of the steam discharged from the pressure vessel to the turbine that can be measured at the reactor dome or at the inlet to the turbine, neutron flux or core power, feedwater temperature and flow rate, steam flow rate provided to the turbine and various status indications of the BWR systems. Many monitoring parameters are measured directly by conventional sensors, while others, such as core thermal power, are conventionally calculated using measured parameters. Output from the conventional sensors and calculated parameters are input to an emergency protection system to assure safe shutdown of the plant, isolating the reactor from the outside environment if necessary, and preventing the reactor core from overheating during any emergency event.
Conventional pressure control of the BWR is provided by automatically adjusting the position of the main turbine control valves, or steam admission flow control valves to the turbine. The control system must maintain control valve position margin below valves-wide-open (VWO) so as to provide adequate reactor pressure control should the pressure rise for any reason. If the reactor pressure rises, the steam admission control valves will open beyond the initial position, thus restoring the reactor system pressure to its desired value. For a conventional pressure control system, the margin in steam flow between the normal desired operating point of the steam admission flow control valves compared to the steam flow where the steam admission flow control valves are wide open is required to be about 3% of rated steam flow to maintain adequate performance.
The main turbine control valves are controlled by a pressure regulation system and valve servo system which position the turbine inlet flow control valves. Also, several steam bypass valves are included in the plant design. These bypass valves are used for plant startup and to bypass excessive steam should the need arise. The pressure regulator uses system pressure as one input and pressure setpoint as the second input. Each of the main turbine control valves is typically controlled by a control valve servo loop which has a flow demand to valve position demand characterizer and the actual valve position as inputs to the control valve servo loop. The bypass valves are typically controlled by a similar servo loop. The bypass valves and in some cases, the main control valves are opened in a planned sequence according to steam flow demand needs.
The current BWR reactor system pressure regulation requires the main turbine control valves to change position or modulate to maintain reactor system pressure. As noted above, when the reactor pressure decreases, the control valves close to restore reactor system pressure to the desired value, or conversely, if the reactor system pressure increases the control valves open to reduce reactor system pressure to the desired value. As an example, for many BWR types of plants, the main turbine valves are typically operated in full arc mode, i.e., all turbine flow control valves move together, with average position near 50% of wide open. Control valve modulation is around this average valve position. If operation greater than about 60% valve position is attempted, the pressure control system will become less effective and steady plant operation can not usually be maintained. Other BWR plants operate in partial arc mode in which the turbine control valves are opened in a planned sequential order. In partial arc mode, conventional pressure control at full power is primarily accomplished with all but one turbine control valve wide open. The last turbine control valve modulates at a partially open position, typically about 30% of wide open. When the main turbine control valves are operating near their normal full power position, i.e., 50% open in full arc mode, the turbine control valves are passing less steam flow to the main turbine than if the valves were wide open for the same system pressure, and as a result less electrical output is generated. It would be desirable to operate a BWR plant under conditions that maximize electrical output and still maintain reactor system pressure within acceptable limits.